This invention relates to a color image reading apparatus in a color facsimile system or color copying machine, in which a color image is converted into an electrical signal, so as to minimize the unevenness in color reproducibility of such apparatus.
FIG. 6 is a block diagram showing the arrangement of a conventional color image reading apparatus described, for instance, in the literature "NEC Technical Report Vol. 41, No. 3/1988". In FIG. 6, reference numeral 18 designates a CCD sensor with color filters for converting light reflected from a color original into electrical signals according to the colors; 19, a drive amplifier for amplifying the output electrical signals of the CCD sensor 18; 20, an A/D (analog-to-digital) converter for converting the output analog electrical signal of the drive amplifier 19 into a digital signal; 21, a signal processing circuit for performing signal processing operations such as shading correction and noise reduction; 22, an image reducing circuit for reducing an obtained image in compliance with a process done in the later stage; 23, a contour emphasizing circuit for emphasizing a contour part to improve the visual picture quality of a given image; 24, a ring buffer circuit for operating with a line memory circuit 29 which is provided for a pipe line process in order to process an image at high speed; 25, a color correcting circuit for correcting color signals according to correction coefficients which have been set in a table memory circuit 28; 26, an error diffusing circuit for applying a binary process such as error diffusion to a multi-valued image signal in response to a requisition from a printer or the like; and 27, a control circuit for controlling a series of operations from the processing of the signals read from the color original with the CCD sensor 18 up to the transmitting of the signals through the error diffusing circuit 26.
The operation of the conventional color image reading apparatus thus organized will be described.
The analog image signal, which the CCD sensor 18 has read from the color original, is amplified by the drive amplifier 19, and converted into multi-valued digital data by the A/D converter 20. The image signal thus processed is applied to the signal processing circuit 21, where noises are removed from the image signal, so that the latter signal is converted into a signal uniform in distribution without such as shading. In compliance with a requirement in aftertreatment, the output signal of the signal processing circuit is applied to the image reducing circuit 22, where it is subjected to image reduction, and the output signal of the image reducing circuit 22 is applied to the contour emphasizing circuit 23, where it is subjected to contour emphasizing. In order to correct the color signal thus obtained according to the color temperature of the light source and the color shift of a color filter in the reading of the original with the CCD sensor 18 or the color reproducing tendency of a printer or display in the aftertreatment system, to obtain an excellent color reproducibility, the output signal of the contour emphasizing circuit 23 is applied through the ring buffer circuit 24 to the color correcting circuit 25. In the color correcting circuit 25, a color correcting operation is carried out using correction coefficients which have been stored in the table memory circuit 28 in compliance with the actual conditions of the color image reading apparatus. The output of the color correcting circuit 25 is applied to the error diffusing circuit 26, where it is subjected to error diffusion, so that it is output as binary image data to an external printer or the like. The ring buffer circuit 24 cooperates with the line memory circuit 29 so that the image signal continuously transmitted thereto is subjected to pipe line process continuously, thus permitting a high speed operation.
The general tendency of the color image reading apparatus thus organized is such that, as was described before, the light source, the color separating filter, the CCD sensor's spectral sensitivity, and the color reproducing characteristic of output equipment such as a printer or display unit are not always ideal with the apparatus. Hence, it is necessary to subject the image data read to color correction, to improve the color reproducibility. In order to provide the ideal color reproducibility, it is necessary to store color correcting coefficients in the table memory in compliance with the actual conditions of those parts in advance. That is, the image signal read by the CCD sensor 18 is subjected to color correction in the color correcting circuit 25 using the correcting coefficients thus stored.
It is assumed that the original signals obtained through the CCD sensor 18 and the A/D converter 20 are represented by RI, GI and BI, respectively, and that the color signals corrected by the color correcting circuit 25 are represented by RO, GO, and BO, respectively (where R, G and B being primary colors "Red", "Green", and "Blue", respectively. In this case, the color correction is carried out according to the following expression (1): ##EQU1##
where a.sub.ij (i=1 through 3, and j=1 through 3) is a correcting coefficient. The correcting coefficients are calculated and stored in the table memory circuit 28 in advance. In the color correcting circuit 25, its hardware carries out the color correcting operation using the correcting coefficients stored in the table memory circuit.
However, the different image reading apparatuses have different spectral characteristics due to their light sources, color separation filters, CCD sensors, display units, printers, etc. Therefore, application of the color correcting coefficients calculated in advance to a plurality of image reading apparatuses cannot make them coincident in color reproducibility with one another.
Further, in a multi-chip image sensor having a plurality of image sensor units, its different image sensor units have different spectral characteristics and, therefore, it is rather difficult to make them coincident in color reproducibility with one another.
Furthermore, in the case where the spectral characteristic changes positively with time as in the case of a light source or color separating filter, the color reproducibility cannot be improved with the correcting coefficients stored in the table memory circuit 28. This is a serious problem. This difficulty may be eliminated by revising the contents of the table memory circuit 28 in accordance with the actual conditions of the apparatus concerned. However, the operation of calculating new correcting coefficients for an apparatus given and writing them in the table memory circuit 28 is nearly impossible after the apparatus has been used.
As was described above, in the conventional color image reading apparatus, the color correcting coefficient is calculated outside the apparatus and stored in the memory in advance. Therefore, in the case where color image reading apparatuses are different in color reproducibility from one another, the adjustment cannot be made. Furthermore, it is difficult to rewrite the correcting coefficient. In the case where the apparatus has a plurality of image sensor units, it is difficult to eliminate the unevenness in characteristic of the image sensor units.